1. Field of the Invention
The present invention relates to a measuring method, an optical scanning device, and an image forming apparatus, and more particularly to a measuring method for measuring a deviation in timing of start of writing in a plurality of scanning lines, an optical scanning device including a plurality of light-emitting elements, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Image forming apparatuses such as laser printers and digital copiers have been required to perform image formation at a higher speed (increased speed) and to have a higher writing resolution (improved image quality). To meet these requirements, a technology for scanning a surface to be scanned with a plurality of light beams using a multi-beam light source has been proposed.
In a multi-beam light source including a plurality of light-emitting elements, the light-emitting elements are arranged so as to be displaced from one another in a direction corresponding to a main-scanning direction, so that the space between the scanning lines can be reduced. In such an arrangement, in order to align the positions of start of writing in a plurality of scanning lines, there is a need to control timing at which each of the light-emitting elements is turned on.
For example, Japanese Patent Application Laid-open No. 2003-276234 discloses an image forming apparatus having a control unit that determines the timing at which at least one of light-emitting elements in a light source unit is turned on, based on an image clock generated in synchronization with a detection signal from a synchronization detecting unit, and determines the timing at which the other light-emitting elements are turned on by changing a count of the image clock, and control turning on the light emitting elements in the light source unit.
After the image forming apparatus is shipped, optical characteristics of its optical components or the positional relationship between these optical components could change due to a temperature change or aging. Because of such a change, positions of light spots on the surface to be scanned could be displaced from desirable positions. This causes a displacement between positions of start of writing of image data with respect to the main-scanning direction, for example. It is difficult for the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2003-276234 to correct such a displacement between the positions of start of writing in real time and precisely.